1. Field of the Invention
The present invention relates to a percussion hand-held tool, e.g., a chisel hammer or a hammer drill, with an electromagnetic hammer mechanism.
2. Description of the Prior Art
In hand-held tools with an electromagnetic hammer mechanism, a striking piston, which imparts axial blows to a working tool primarily via an intermediate piston, reciprocates under an action of timewise changeable magnetic field.
U.S. Pat. No. 4,215,297 discloses a hand-held tool in which a striking piston, which is supported for a limited axial movement relative to the working tool, cooperates with a ferromagnetic insert coaxially arranged inside of a coil that produces an intermittently changeable magnetic field. The striking piston is connected with a storage spring for storing the recoil energy. The drawback of such an arrangement consists in a poor electrical efficiency of the percussion or impact energy that can be achieved. The increase of the electrical efficiency is only possible with an unacceptable decrease of the service life of the hand-held tool.
German Publication DE 198 39 464A1 discloses a hand-held tool in which an electrodynamic actuator is arranged in a magnetic flux circuit which passes through a permanent magnet, a coil, and an anchor formed of a soft ferromagnetic material. The yoke, here the anchor and the coil, is supported for a limited movement transverse to the magnetic flux, which passes through adjoining surface regions of the U-shaped stator, by springs provided on opposite sides of the yoke. An alternate current passing through the coil can produce constrained oscillations of the obtained spring-mass system. The drawback of this solution consists in that in order to be able to use the above-described striking system as a hammer mechanism for a hand-held tool, piston-forming components should have large masses. Another drawback of this solution is a need to provide additional elements for feeding current to the movable coil, which reduce the service life of the hammer mechanism.
In the hand-held tool disclosed in WO 9940673 A1, an oscillating linear drive functions in accordance with a principle of a polarized reluctant actuator (PRA). In the disclosed hammer mechanism, a yoke, here an anchor and a magnet, are displaceably supported in a magnetic flux circuit, with the magnetic flux passing through the permanent magnet, the coil, and the anchor which is formed of a soft ferromagnetic material. The yoke is displaced by a flux passing transverse to the magnetomotive regions of the yoke and having a variable flux density which is varied between both pole shoes by the control magnetic field of a coil of a U-shaped stator, or is displaced by electric field gradients provided in the magnetomotive region in the direction of a minimal total forward resistance. The drawback of the proposed solution consists in that the magnetic and mechanical characteristics of permanent magnets, which are to be used in hammer mechanisms of hand-held tools, are very sensitive to impacts.
An object of the present invention is to provide a hand-held tool with an eletromagnetic hammer mechanism having an increased impact or percussion energy efficiency without any reduction of the service like in comparison with conventional electromagnetic hammer mechanisms.
This and other objects of the present invention, which will become apparent hereinafter, are achieved by providing an electromagnetic hammer mechanism including at least one stator, at least one coil with an axis extending transverse to the oscillation axis of the working tool, a striking piston formed as a yoke supported in a magnetic flux for a limited displacement along the oscillation axis and having at least one magnetomotive region formed of a soft magnetic ferroelectric material, and at least one magnet associated with the stator, magnetizable along the oscillation axis, and arranged along and adjacent to at least one segment of the coil located at an end of the stator.
According to the present invention, as a hammer mechanism, an oscillating linear motor, which is based on a principle of a polarized reluctance actuator (PRA) is used. A magnetic flux-generating magnet is magnetized in the percussion direction of the working tool which corresponds to the direction in which the working tool oscillating axis extends. The magnet is arranged along and adjacent to a coil segment located at an end of the U-shaped stator.
According to the present invention, a pole shoe, which is associated with the segment located at the U-shape stator end, is divided by a gap transverse to the working tool oscillation axis and the coil axis in two half-pole shoes, with the magnetic flux being divided between the two half-pole shoes by a control magnetic field of the energized coil. The yoke, which bridges both pole shoes of the stator and is supported for a limited movement along the working tool oscillation axis, has a magnetomotive region associated with the pole shoes. This region, at least along the working tool oscillation axis, consists of a soft magnetic ferroelectic material. The joke has, with respect to the two half-pole shoes, respective positions along the working tool oscillation axis in which the resulting total magnetic resistance in the gross flux circuit is at a minimum.
In this way, by changing the direction of the current flow through the coil, a bipolar behavior with respect to the anchor position can be realized or, the forces, which can be used for producing oscillations, can be applied to the yoke, which forms an anchor of a linear motor and which is located between the two positions, in both directions. The linear motor anchor functions as a striking piston and because the striking piston-forming anchor can have a small mass, only light vibrations of the hand-held tool are generated.
Advantageously, the strength of the coil current is selected in accordance with the condition of the increase of the flux, which is generated by the magnet, in a half-pole shoe. This permits to double the flux in another half-pole shoe. In this way, the coil provides for an optimal controlled behavior of the anchor.
Advantageously, the anchor has its end surface, remote from the working tool, connected to the hand-held tool housing by a storage spring for storing the recoil energy. The resulting energy-storing, self-oscillating system further increases the efficiency of the inventive hammer mechanism.
In accordance with a further advantageous embodiment of the present invention, a pole-shoe, which is divided by a gap into two half-pole shoes, is associated with each of two opposite segments of the coil provided at respective ends of the stator having a U-shape cross-section in the longitudinal direction. The anchor is formed with two magnetomotive regions associated with respective pairs of half-pole shoes and extending in the direction of the half-pole shoes. The anchor is so formed that it has a minimal magnetic resistance in the magnetomotive regions. As a result, the forces, which are generated due to the energization of the coil and by apportioning of the flux, are added to each other. With mirror-symmetrical arrangement of the system in a plane perpendicular to the working tool oscillation axis, the forces are doubled.
Advantageously, a spacer, which is provided between the two magnetomotive regions of the anchor, can be formed with a small mass, while maintaining an adequate stability. This permits to reduce the mass of the striking piston, by preferably symmetrical narrowing of the anchor, i.e., of the yoke anchor cross-section. However, care should be taken to maintain a minimal necessary flux cross-section.
In accordance with a further advantageous embodiment of the present invention, a further stator, which has a longitudinal U-shaped cross-section and which is arranged rotationally symmetrically with the first stator with respect to the working tool oscillation axis, is associated with the common, anchor. The further or second stator is associated with further magnetized magnet, half-pole shoes, and segments of another energized coil. The provision of the second stator system increases the forces acting on the anchor. When the second system is arranged symmetrically with respect to the first system, these forces are again doubled.
Advantageously, with respect to both longitudinally U-shaped stators, the current flows through both coils in the same direction, but the two magnets, which are associated with respective coils are magnetized in opposite directions. As a result, with the symmetrical arrangement of the four half-pole shoes pairs, the flux in the anchor between the two magnetomotive regions arranged along the oscillation axis, increases. Therefore, no measures are needed to maintain a minimal flux cross-section of the magnetomotive regions, and a less thick spacer between the two magnetomotive regions can be used as its magnetic characteristics can be disregarded.
While capable to withstand high mechanical alternating loads, the, advantageously, one-piece anchor, which forms the striking piston, can be formed with a relatively small mass, forming a composite striking piston with a spacer formed of a light material.
The magnetic flux circuit is closed through both U-shaped stators and both, axially spaced along the oscillation axis, magnetomotive regions of the anchor. The flux passes through the magnetomotive regions in a direction transverse to the oscillation axis, which permits to maximize the force acting on the anchor.
Advantageously both coils are partially curved about the oscillation axis, which permits to reduce the space requirement, while retaining the same power.
Advantageously, the striking piston-forming anchor is formed with flat surfaces in the direction the flux passes therethrough. The anchor is formed as a mirror-symmetrical part, with rising, relative to the cross-section, surfaces of the magnetomotive regions. The transversely extending side edges serve advantageously for positioning of the striking pistion-forming anchor transverse to the oscillation axis, and as guide means parallel to the oscillation axis.
With the advantageously rhombic cross-section of the flat striking piston-forming anchor or yoke, the portions of the magnetomotive regions, which form an acute angle and which define the side edges, serve far positioning or guiding the corresponding, associated angularly-shaped pole shoes in the U-shaped stators. To prevent friction and the resulting wear, thin, slidable, preferably magnetic gap-forming, non-magnetic shims are placed on the pole shoes and/or the respective portions of the magnetomotive surfaces.
Advantageously, an end surface of the striking piston-forming anchor or yoke adjacent to the working tool, is provided with a radial circular surface which is so formed that it transmits blows to the working tool or an intermediate piston with a minimum wear.
Advantageously, the end surface, adjacent to the storage spring, of the striking-piston forming anchor or yoke is provided with at least partially radial annular surface for forming at least partial circumferential contact with the storage spring. Advantageously, in order to reduce weight, a blind bore is formed within this surface.
Advantageously, a preload spring applies a biasing force to the end surface of the striking piston-forming anchor or yoke adjacent to the working tool. As a result, a steady operation of both springs in the region of alternating loads and in the region of oscillating loads in insured. This increases the service life of both spring.
The novel features of the present invention which are considered as characteristic for the invention, are set forth in the appended claims. The invention itself, however, both as to its construction and its mode of operation, together with additional advantages and objects thereof, will be best understood from the following detailed description of preferred embodiments when read with reference to the accompanying drawings.